1. Field of the Invention
The present invention relates generally to magnetic pumps, and more particularly, to a magnetically suspended pump for use in medical equipment such as artificial heart-lung machine and artificial heart. The magnetically suspended pump supports an impeller by a magnetic bearing and rotates the impeller by a motor through a partition by means of magnetic coupling.
2. Description of the Background Art
FIGS. 4A and 4B are views showing a conventional magnetically suspended blood pump. In FIG. 4A, an impeller 3 is provided in the casing 2 of a magnetically suspended blood pump 1. Casing 2 formed of a nonmagnetic material includes a nonmagnetic member 5 having permanent magnets 4 which forms a passive magnetic bearing and a soft iron member 6 which corresponds to the rotor of an active magnetic bearing. Permanent magnets 4 are divided along the circumference of impeller 3, and adjacent magnets are polarized in the opposite directions to one another.
On the side opposite to the side with the permanent magnets 4 of impeller 3, a rotor 7 is provided separated from impeller 3 by casing 2. Rotor 7 is driven to rotate by a motor 9. Rotor 7 is provided with permanent magnets 8 of the same number as and opposite to the permanent magnets 4 of impeller 3 such that the attracting force is effected between them.
Meanwhile, electromagnets 10 and position sensors S1 to S4 are provided opposite to the side of the soft iron member 6 of impeller to retain impeller 3 in the center of casing 2 against the attracting force between permanent magnets 4 and 8. Position sensors S1 to S4 are provided between the coils M1 to M4 of electromagnets 10 as shown in FIG. 4B.
In magnetically suspended blood pump 1 having the above-described structure, permanent magnets 8 incorporated in rotor 7 drive and radially support impeller 3, and cause the axial attracting force between permanent magnets 4 provided to impeller 3 and them. Current corresponding to the attracting force is passed through the coils of electromagnets 10, which causes impeller 3 to be suspended. Then, rotor 7 is rotated by the driving force of motor 9 to generate magnetic coupling between permanent magnets 4 and 8, which rotates impeller 3, and blood is introduced from an inlet port to an outlet port which are not shown. Impeller 3 which is separated from rotor 7 by casing 2 is not contaminated by electromagnets 10, and therefore blood discharged from magnetically suspended blood pump 1 maintains its clean state. FIG. 5 is a diagram showing a control circuit for the magnetically suspended blood pump as shown in FIGS. 4A and 4B. The Z-axis, .theta..sub.x -axis and .theta..sub.y -axis are produced by operations according to the following expressions using an operation circuit (not shown) based on signals from position sensors S1 to S4 shown in FIG. 4. ##EQU1## wherein r is the radius of electromagnet 10.
The values of Z, .theta..sub.x and .theta..sub.y produced as above are input to PID (Proportional integral Derivative) compensators 11 to 13 shown in FIG. 5, the output of PID compensator 11 is applied to one input end of each of adders 16 to 19. The output of PID compensator 12 is applied to the other input end of adder 16, also added with a coefficient by a coefficient unit 14 and applied to the other input end of adder 18. The output of PID compensator 13 is applied to the other input end of adder 19, also added with a coefficient by a coefficient unit 15 and applied to the other input end of adder 17. Adders 16 to 19 add the respective two inputs, and apply the results to amplifiers 20 to 23. Amplifiers 20 to 23 drive corresponding coils M1 to M4.
The magnetically suspended blood pump 1 shown in FIGS. 4A and 4B requires the four coils M1 to M4 and four position sensors S1 to S4 for electromagnets 10 which constitute the active magnetic bearing. Although reducing the number of such coils and position sensors for electromagnets 10 naturally reduces manhour involved and improves the reliability, but on the other hand if the number of coils and sensors are reduced to three, the circumference of each magnet will be longer, and .theta..sub.x and .theta..sub.y cannot be accurately controlled.